<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #box1{
            width: 100px;
            height: 100px;
            background-color: blue;
            margin-left: auto;
            margin-right: auto;
        }
    </style>
</head>
<body>
<div id="box1">

</div>
</body>
</html>